This device is an ordinary indicator, but it's also equipped with rechargeable batteries.
At a power failure of mains
will these rechargeable batteries supply the phone line indicator. When the main supply
current is returning, the
batteries will be charged.
At a power failure I've measured this current supplying time:
48 hours or more
T: picture 2, top
Current limiter circuit
The potentiometer P is set in that way the transformer will be never overloaded
by the charging current.
V40-41 = 1.1 volts
